A major goal of the project is the elucidation of factors regulating blood flow in the renal medulla. Among the questions we wish to answer are: does the blood flow vary with the diuretic state of the animal, and if it does, is control exerted in the afferent arteriole, efferent arteriole, or in the vasa recta? Another question is whether there is autoregulation of inner medullary blood flow, because early studies suggested there was not. To study these questions, we will implement a video based modification of the dual slit method for estimating erythrocyte velocity. The method has recently been calibrated for circumstances typical of the renal medulla; it permits continuous estimation of velocity and radius in several vasa recta, and these variables will be monitored in states of antidiuresis, water diuresis, saline diuresis, and with alterations of arterial blood pressure. In another study, we will use a digital image processing system in conjunction with FITC labelled dextran to measure flow rate of tubular fluid in response to transient changes in glomerular filtration rate. In a third study, we will perfuse loops of Henle in rats with an immune lesion of the glomerulus to estimate the efficacy of macula densa feedback in adjusting glomerular permeability to water.